FIG. 9 shows a demodulator 1 according to the prior art that is used to determine and select symbols D from a digitalized signal sd, which consist of a plurality of individual components and which are coupled with a quadrature signal pair according to a modulation procedure, for instance according to the QAM standard. In the embodiment illustrated in the figure, the demodulator 1 receives at an input an analog signal sa from a signal source. This analog signal sa is supplied to an A/D converter 2 for conversion to a digital signal. In addition, the A/D converter 2 is also equipped with an input for a sampling signal t. The digital signal sd is supplied by the A/D converter 2 to a quadrature converter 3. The quadrature converter 3 converts the digital signal sd in the baseband and outputs digital signal sd which is split into both quadrature signal components of the Cartesian coordinate system. In order to convert the frequency, the quadrature converter 3 is fed two carriers offset by 90° from a local oscillator 15, whose frequency and phase is controlled by a carrier control device 14. The quadrature signal components are supplied from the quadrature converter 3 to an amplification control device 4.
An output signal of the amplification control device 4 is supplied to a filter 5. Both quadrature signal components I, Q are then supplied to a symbol sampling device 6, which is equipped with sampling control. The control of the symbol sampling device 6 is performed through an input to which is supplied sampling signal ti. The symbol sampling device 6 applies a temporal interpolation between the real sampling values at the symbol rate or at a whole number multiple thereof. As an alternative to this, the A/D converter can be controlled with the sampling signal. In this case, the digital signal would be provided already at the symbol rate or a multiple thereof and the sampling device could be eliminated. The output signal of the sampling device 6 is filtered, in particular by means of a Nyquist filter 7, and supplied to an equalizer 8. The equalizer 8 makes available a temporary symbol at its output. Thereafter, adjusted symbols D are formed by a symbol discriminator 10. These symbols D are then furnished to further digital processing devices. Thus, in this manner, the discriminator 10 extracts digital data.
This discriminator 10 is connected to provide discriminator feedback for control of the carrier frequency/phase, the sampling timing, the equalizer. The clock control device 13 generates the sampling signal ti, which is supplied in particular to the symbol sampling device 6 or to the AD converter in the alternative embodiment. In addition, the clock control device 13 supplies a signal to the equalizer 8. The carrier control device 14 applies a phase difference or a phase offset ΔΦ, which is determined in a rotation control device 12. The phase offset ΔΦ is determined between the phase Φ of the symbol before the discriminator 10 and the phase Φd of the symbol D after the discriminator 10.
This type of a device and method for QAM frequency control provided with a discriminator are known from DE 36 19 744 or DE 103 44 756.
For the transmission of digital signals with QAM, receiver designs using complex downwards mixing into the baseband, as well amplitude control and carrier frequency/carrier phase and sampling control, are typically employed. The detection of the carrier frequency and phase is difficult to achieve with a PLL (phase-locked loop), in particular with high-performance modulations, because mostly erroneous determinations will be carried out during the acquisition phase. The phase offset ΔΦ is the angle differential between the sampled and digitalized received signal, and the determined symbol of the alphabet. A frequency offset results in a constant modification of a correctly determined phase offset and can be measured as the difference between successive phase offsets.
A conventional QAM receiver mixes, for example, using a circuit such as the circuit shown in FIG. 9, the signal which is found on a carrier frequency in the baseband and samples it according to timing for symbols, wherein a decision is made after a level adjustment, Nyquist filtering and adaptive filtering have been carried out in an equalizer. The sequence of the functional blocks that is used in this case to carry out the mixing into the baseband, or the sampling with the symbol rate and amplification control, is irrelevant. The mixing into the baseband can be achieved in the analog region before the subsequent complex, or two-channel, A/D conversion, or after the A/D conversion of the intermediate frequencies, and in an alternative, the sampling to symbol points in time can have already been provided by the A/D converter, or, when this converter operates with asynchronous operation on any frequency, with a digital sample conversion, and blocks for amplification control can be added in principle at any stage.
With a simple QAM receiver, the control of the local oscillator must also stabilize the carrier phase.